The proposed study will launch a multidisciplinary effort to examine the diffusion of HIV and sexually transmitted diseases (STDs including gonorrhea, chlamydia, and syphilis) in drug users in L.A. County to identify the individual level factors, partner-level factors, and environmental factors that promote the spread of these diseases. L.A. County is the second largest epicenter of AIDS cases in the nation, yet injection drug use (IDU) accounts for only a minority (13%) of cases, while the majority of cases involve MSM (70%). Recent, disproportionate increases of HIV infection for women and people of color in L.A. County imply the virus is moving from relatively contained groups into larger segments. This 5-year study proposes to establish a representative cohort of individuals thought to represent the behavioral "bridges" for these pathogens to enter the larger population: drug using MSM (n=240), drug using MSM/W (n=240) a comparison group of non-drug using MSM/W (n=240), and the male (n=288) and female (n=192) sexual partners of these individuals (total=1,200). Assessments will be collected at baseline, 6- and 12-months after enrollment. Data collected will address these study aims: (1) Measure associations between drug involvement (methamphetamine user, other drug user, non-drug user), IDU status, sexual risk behavior (MSM, MSM/W, WSM), and HIV/STDs; (2) Evaluate the types of sexual partnerships and dynamics of the partnerships of these individuals and how these are associated with HIV/STD transmission; and (3) Apply mathematical models to the data on partnerships to study how the incidence of HIV/STDs reflect the size and interconnectedness of the sexual networks of each of the groups and to determine the impact of sexual network structure in future transmission of HIV in L.A. within and beyond MSM and heterosexual drug using groups. The study will use methods of behavioral epidemiology, ethnography, viral analysis of HIV, and mathematical modeling to yield a comprehensive set of information to predict the spread of HIV and STDs from sexual networks of high HIV prevalence (drug using MSM) to those of low prevalence (heterosexuals). Outcomes from the proposed cohort study of IDU and non-lDU methamphetamine-using MSM and MSM/W and their sexual partners should provide evidence to guide policy and prevention efforts in response to the spread of HIV and STDs.